1. Field of the Invention
The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is concerned with a top nozzle allowing improved utilization of space for accommodating greater thermal growth and burnup of fuel rods of the fuel assembly.
2. Description of the Prior Art
In most nuclear reactors, the reactor core is comprised of a large number of elongated fuel assemblies which receive support and alignment from upper and lower transversely extending core support plates. The upper and lower core support plates are directly or indirectly attached to a support barrel which surrounds the entire core and extends between the ends thereof.
Conventional designs of these fuel assemblies include a plurality of fuel rods and control rod guide thimbles held in an organized array by a plurality of grids spaced along the fuel assembly length and attached to the control rod guide thimbles. The guide thimbles extend slightly above and below the ends of the fuel rods. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the guide thimbles to thereby form an integral fuel assembly. The fuel assemblies are arranged vertically resting on the lower core support plate. To facilitate handling and installation, the fuel assemblies are generally not secured to the lower core support plate.
Temperatures at various times within the core may vary greatly, such as, from cold shutdown to normal operating conditions. Also, different materials exhibit different thermal growth characteristics. Since the materials of fuel assembly components are generally different than those used in the core support barrel and undergo greater thermal expansion, the resulting increase in length of the fuel assemblies in the axial or vertical direction must be accommodated. For this reason, the fuel assemblies are not usually attached to the upper and lower core plates but rather are supported in a manner which permits some relative motion therebetween.
The axial thermal expansion differential between the fuel assemblies and the core support barrel has been accommodated by insuring that the axial spacing between the upper and lower core support plates is somewhat greater than the axial length of the fuel assemblies. Normally, this is accomplished by providing an axial gap between the top of the fuel assemblies and the upper core support plate. However, the presence of the gap can result in upward lifting of the fuel assemblies due to the hydraulic forces produced on the fuel assemblies in the upward direction by coolant flow. Thus, fuel assemblies have also employed hold-down devices with the top nozzles to prevent the force of upward coolant flow from lifting the fuel assemblies into damaging contact with the upper core support plate, while at the same time allowing for changes in fuel assembly length due to core-induced thermal expansion and the like. Representative of prior art fuel assemblies with hold-down devices are those disclosed in Hatfield (U.S. Pat. No. 4,792,429) and Wilson et al (U.S. Pat. No. 4,684,502) and Japanese Pat. Nos. 62-91891 and 62-102186.
As mentioned previously, the guide thimbles of fuel assemblies extend slightly above and below the ends of the fuel rods. Thus, the top and bottom nozzles of fuel assemblies secured at opposite ends of the guide thimbles likewise are spaced above and below the fuel rod ends. This space between the opposite ends of the fuel rods and adjacent portions of the top and bottom nozzles accommodates increase in length of the fuel rods due to thermal growth as fuel rod burnup occurs during normal reactor operation.
With improvements in various aspects of fuel assembly design, it has become feasible to increase the allowable burnup of the fuel rods. This increase in burnup is desirable because it decreases the frequency of plant shutdowns and the buildup of spent fuel. However, to permit the fuel rods to operate to a higher burnup, an increase of approximately 0.5 inch minimum in fuel rod length is necessary due to extra thermal growth. This necessitates an increase in the space between the adapter plates of the top and bottom nozzles to accommodate this additional fuel rod growth. At the same time, there still must be enough space between the upper core plate and adapter plate of the top nozzle to allow inserting and latching of the handling equipment to the top nozzle.
Currently, there is not enough room between the adapter plates of the top and bottom nozzles to permit the 0.5 inch growth in fuel rod length. Consequently, a need exist for a way to accommodate extra fuel rod thermal growth without impairing the handling capability of the core equipment currently in use.